As of now, liquid crystal display devices under development which have wide viewing angle characteristics includes liquid crystal display devices utilizing the IRS (In-Plane-Switching) mode which is a transverse electric field mode or the FFS (Fringe Field Switching) mode, and liquid crystal display devices utilizing the VA (Vertical Alignment) mode. The VA mode is better for mass production than the transverse electric field mode and is therefore used in a wide range of TV applications and mobile applications.
The VA mode liquid crystal display devices are generally classified into MVA (Muitidomain Vertical Alignment) mode liquid crystal display devices, in which one pixel includes a plurality of domains of different liquid crystal alignment directions, and CPA (Continuous Pinwheel Alignment) mode liquid crystal display devices in which the liquid crystal alignment direction continuously varies around a rivet or the like formed on an electrode at the center of a pixel.
In the MVA mode liquid crystal display devices, the alignment control means which extend in two mutually-orthogonal directions are provided to form four liquid crystal domains in one pixel, in which the azimuthal angles of the directors representing the liquid crystal domains are 45° relative to the polarization axes (transmission axes) of a pair of polarizing plates in a crossed nicols arrangement. Assuming that the direction of the polarization axis of one of the polarizing plates is azimuthal angle 0° and that the counterclockwise direction is the positive direction, the azimuthal angles of the directors of the four liquid crystal domains are 45°, 135°, 225°, and 315°. Such a structure which includes four domains in one pixel is referred to as “four-division alignment structure” or simply “4D structure”. When, in each of the four domains, the alignment direction and the polarization axis of the polarizing plate form an angle of 45°, the change in retardation in a liquid crystal region can be utilized most efficiently.
The MVA mode liquid crystal display devices are not suitable to small pixels (for example, the shorter side is less than 100 μm, particularly less than 60 μm). For example, when slits (or ribs) are used as the alignment control means, the width of the slits need to be about 10 μm or more in order to obtain a sufficient alignment control force. If the slit width is narrower than this, sufficient alignment control force cannot be obtained. To form four domains, it is necessary to form, in a counter electrode, slits (“<”-shaped slit) extending in directions different by 90° from each other when seen in a direction normal to the substrate and to form, in a pixel electrode, slits which are separated by a certain space from the counter electrode slits and which extend parallel to the counter electrode slits. Specifically, both the counter electrode and the pixel electrode in one pixel need to have a plurality of slits extending in the direction of 45°-225° and the direction of 135°-315° and having the width of about 10 μm.
However, when the above-described slits are applied to a pixel whose shorter side is less than 100 μm, the ratio of the area of the slits to the pixel area increases, and accordingly, part of the pixel area which fails to contribute to display increases, so that the transmittance (brightness) significantly decreases. In the case of a small-size liquid crystal display device of finer definition, e.g., 2.4-inch VGA for use in mobile phones, the pixel pitch (row direction×vertical direction) is, for example, 25.5 μm×76.5 μm. In such a small pixel, the above-described slits cannot be formed.
In the CPA mode liquid crystal display devices, a rivet is formed of a resin or the like in the counter electrode at the pixel center, such that the rivet and a diagonal electric field produced at an edge of the pixel electrode serve to regulate the alignment of the liquid crystal. Provided in the respective gaps between the two polarizing plates and the liquid crystal layer are ¼-wave plates (quarter wave plates). By utilizing omniazimuthal, radial slope alignment domains and circular polarization, high transmittance (brightness) can be achieved.
The CPA mode which utilizes the ¼-wave plates achieves high transmittance but disadvantageously provides a low contrast ratio and a narrow viewing angle as compared with the MVA mode. Specifically, when the ¼-wave plates are used, the display (especially, the display at lower gray levels (lower brightness)) appears brighter, i.e., so-called “whitish dots” are conspicuous, when observed in a diagonal viewing angle than when observed in front of the display surface (when observed in a direction normal to the display surface (viewing angle 0°)).
To solve the above problems of the liquid crystal display device in the MVA mode and the CPA mode, liquid crystal display devices as disclosed in Patent Document 1, Patent Document 2, and Patent Document 3 have been proposed. In the liquid crystal display devices of these patent documents, the four-division alignment structure is realized by forming in the pixel electrodes a large number of narrow slits extending in the direction of 45°-225° and in the direction of 135°-315° (referred to as “fishbone pixel electrode”) such that the liquid crystal is aligned parallel to the slits. In liquid crystal display devices which use such fishbone pixel electrodes, large slits or rivets are not formed in pixels, and linearly-polarized light is used without using ¼-wave plates. Therefore, display can be realized with high transmittance, high contrast ratio, and wide viewing angle.
Note that the liquid crystal display devices of these patent documents include alignment sustaining layers on surfaces of the upper and lower substrates on the liquid crystal layer side for making the liquid crystal have an appropriate pretilt angle during absence of voltage application to the liquid crystal. These alignment sustaining layers are formed by polymerizing monomers contained in the liquid crystal layer during application of a voltage to the liquid crystal.    [Patent Document 1] Japanese Laid-Open Patent Publication No. 2003-255305    [Patent Document 2] Japanese Laid-Open Patent Publication No. 2003-149647    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2006-330638